Atrial synchronous pacemakers are designed for use on patients whose hearts have normal atrial self pacing, but, due to a defect in the conduction from the atrium to the ventricle, the ventricles fail to beat or keep pace with the atrial rhythm. Atrial synchronous pacemakers are designed to sense the naturally occurring atrial contractions (depolarizations) and, at the end of a short time interval, to provide an electrical stimulation pulse to the ventricles of the heart so as to induce a ventricular contraction. The time delay interval is selected so that the atrial and ventricular contractions are synchronized with an appropriate delay interval for efficient pumping. Atrial synchronous pacing attempts to take the place of missing natural conduction of stimulating pulses from the atrium to the ventricle in the heart, while the heartbeat rate is free to follow, within limits, the natural rhythm established by the atrial self-pacing of the patient. Often other features are combined with atrial synchronous pacing, such as an upper rate limit, or reversion to fixed rate pacing if the spontaneous atrial rate drops below a predetermined rate. Also, provisions may be provided as is known in the art for programming the pacemaker after implantation to adjust the atrial-ventricular delay, upper or lower rates, and other operating parameters. A ventricular inhibit function can also be provided so that if a ventricular depolarization does follow in response to an atrial depolarization, the ventricular depolarization will be sensed and the pacemaker will be inhibited from delivering a competitive ventricular stimulating pulse. In that manner, the stimulating pulses are delivered only if needed.
In addition to atrial synchronous pacemakers, other types of pacemakers capable of operating in a number of modes include atrial synchronous operation as one possible mode of operation. Dual sense/dual pace atrial-ventricular pacemakers, sometimes referred to as fully automatic pacemakers, are capable of selectively delivering stimulating pulses to both the atrium and the ventricle, and are also capable of sensing beats occurring in both chambers and operating as appropriate to maintain A-V synchrony. That type of fully automatic pacemaker will operate in an atrial synchronous mode if the patient's atrium is self pacing above the minimum rate (thus inhibiting delivery of atrial stimulation pulses) and if the ventricles are not contracting on their own at the proper time interval following an atrial contraction. In that case, a ventricular stimulation pulse will be delivered at the selected A-V delay interval following the atrial contraction, thus resulting in atrial synchronous operation.
Pacemakers operating in an atrial synchronous mode are subject to certain errors which, under certain circumstances, can lead to the delivery of a ventricular stimulation pulse at an inappropriate and possibly dangerous time period of the heartbeat cycle. The problem can occur when a premature ventricular contraction occurs prior to an atrial contraction. Although the atrial sense amplifier is intended to respond only to atrial depolarizations (P-waves) of the electrogram, in fact the R-waves from a ventricular depolarization may be sensed as a P-wave by the atrial sense amplifier. This starts the A-V delay interval, following which the pacemaker delivers a ventricular electrical stimulation pulse. This pulse may fall in time in the vulnerable period of the heart during repolarization from the premature ventricular contraction. Delivery of a stimulation pulse during this vulnerable period is medically unsound, because it could be dangerous to the patient under some circumstances, as it might cause fibrillation of the heart.
It has been recognized in the prior art that the above sequence of events beginning with a premature ventricular contraction and leading to delivery of a ventricular stimulation pulse during the vulnerable period is unacceptable, and various solutions have been proposed. Drug therapy has been used in conjunction with atrial-triggered synchronous ventricular pacemakers in an attempt to suppress premature ventricular contractions so that they would not be detected by the atrial sensing circuit. However, drug therapy has limitations and may be ineffective or inappropriate in certain circumstances and for certain patients. Electronic filtering has also been used in conjunction with the atrial sensing amplifier in order to discriminate the P-wave from the R-wave so as to reject the latter. However, filtration by itself is not workable or reliable in discriminating atrial from ventricular events, because of the inherent variabilities of the shape and frequency composition of R-waves and P-waves in man. The R-wave in some individuals is more like the P-wave in other individuals, thus greatly complicating any attempt at discrimination by electronic filtering.
The present invention overcomes the above noted problem by sensing both in the atrium and ventricle and by using timing considerations to discriminate between P-waves and R-waves so that a stimulating ventricular pulse is delivered only in response to an actual atrial contraction, and not in response to a premature ventricular contraction, when operating in atrial synchronous mode.
The invention is useful in both unipolar and bipolar pacemakers, and is particularly useful in unipolar pacemakers, because in the prior art it has been more difficult to discriminate P-waves from R-waves in the case of unipolar pacemakers. The preferred embodiment shown herein is for a unipolar pacemaker.